The present invention relates to a soybean (Glycine max) seed, a soybean plant, a soybean variety and a soybean hybrid which contain a mutant male sterility gene. This invention further relates to a method for producing hybrid soybean seed and plants.
Male sterility is a condition in plants in which male gametophytic function is prevented, but the potential for female reproduction remains. Based on inheritance patterns, there are two general types of male sterility: 1) genic or nuclear male sterility (gms) and 2) cytoplasmic male sterility (cms). Male-sterile mutations provide source material for studies in plant breeding, genetics, reproductive biology, and molecular biology.
Male sterility has been used in soybean breeding studies (Brim, C. A. et al., Application of genetic male sterility to recurrent selection schemes in soybeans, Crop Sci 13:528-530, 1973; Lewers, K. S., et al., Hybrid soybean seed production: comparison of three methods, Crop Sci (in press), 1996), but so far male sterility has not been used for commercial production of a hybrid seed because large quantities of hybrid soybean seed cannot be produced at the present time. During the past two decades, six genic male sterile mutations (ms1, ms2, ms3, ms4, ms5 and ms6) have been reported in soybean (Palmer, R. G., et al., Male sterility in soybean and maize: developmental comparisons, Nucleus (Calcutta) 35:1-18, 1992). All of these are nuclear mutations inherited as monogenic recessive traits. Cytoplasmic male sterility has not been confirmed in soybean.
Genic male-sterile mutants have been proposed for many crop species breeding programs (Horner, H. T., et al., Mechanisms of genic male sterility, Crop Sci 35:1527-1535, 1995). Controlled production of hybrid seed is necessary for breeding programs and genetic studies. The most feasible methods should utilize close genetic linkage between a male-sterility locus and a seedling marker locus. In soybean, use of the close genetic linkage (Skorupska, H., et al., Genetics and cytology of the ms6 male-sterile soybean, J Hered. 80:403-410, 1989) between a male-sterility locus and a seedling marker locus (W1) is known as the co-segregation method to produce F.sub.1 seeds (Lewers, K. S., et al. Supra). The identification of additional soybean genic male steriles linked to a seedling marker locus would reduce the genetic vulnerability of soybean production of a single genic male sterile.
Marrewijk, G. A. M. V., Cytoplasmic male sterility in petunia I. Restoration of fertility with special reference to the influence of environment, Euphytica 18:1-20, 1969, reported that the phenotypic effect of partial male-sterility systems was subject to environmental modifications. Temperature has more influence than any other environmental factor: however, water stress, photoperiod, nutrients supplied, and hormone applications also influence male sterile phenotypes (Heslop-Harrison, J., The experimental modification of sex expression in flowering plants, Biol Rev 32:38-90, 1957; Edwardson, J. R., Cytoplasmic male sterility, Bot Rev 36:341-420, 1970). In soybean, the msp mutant is affected by temperature (Stelly, D. M., et al., A partially male-sterile mutant line of soybeans Glycine max (L.) Merr.: characterization of msp phenotype variation, Euphytica 29:539-546, 1980; and Carlson, D. R., et al., Effect of temperature on the expression of male sterility in partially male-sterile soybean, Crop Sci 25:646-648, 1985).
The male-sterile soybean mutants ms2 and ms3 result in a degeneration of tetrads because release of microspores from their encasing callose walls is prevented, a phenomenon also described in other, non-lugume, species. For example, the failure of callose to break down at the proper time in cms petunia anthers resulted in sterility (Frankel, R. et al., Timing of callase activity and cytoplasmic male sterility in petunia, Biochem Genet 3:451-455, 1969). The retention of callose seemingly blocks developmental metabolic processes (physical constraints are imposed by the callose wall) and intercellular communication between male cells and locular fluids and between male cells and surrounding tissues.
Abnormal behavior of callase has been observed in several male-sterile systems. Previous studies indicate that the enzyme callase is synthesized in the tapetum, then secreted into the locules, and degrades the callose walls surrounding the microspore tetrads. The timing of production and release of callase by the tapetum, therefore, seems to be critical for normal pollen development (Eschrich, W., Untersuchungen uber den Ab-und Aufbau der Callose, Z Bot 49:153-218, 1961; Frankel, R., et al., Supra; Mepham, R. H., et al., Formation and development of the tapetal periplasmodium in Tradescantia bracteata, Protoplasma 68:446-452, 1969; Izhar, S., et al., Mechanism of male sterility in petunia: the relationship between pH, callase activity in anthers, and the breakdown of the microsporogenesis, Theor Appl Genet 44:104-108, 1971; Stieglitz, H., et al., Regulation of .beta.-1,3-glucanase activity in developing anthers of Lilium, Dev Biol 34:169-173, 1973; Worrall, D. et al., Premature dissolution of the microsporocyte callose wall causes male sterility in transgenic tobacco, Plant Cell 4:759-771, 1992; and Tsuchiya, T., et al., Tapetum-specific expression of the gene for an endo-1,3-glucanase causes male sterility in transgenic tobacco, Plant Cell Physiol 36:487-494, 1995). Premature break down of callose was observed in male-sterile sorghum (Warmke, H. E., et al., Cytoplasmic male sterility in sorghum. I. Callose behavior in fertile and sterile anthers, J Hered 63:103-108, 1972) and in cms petunia (Izhar, S., et al., Supra). Worrall, D., et al., Supra and Tsuchiya, T. et al., Supra reported that a premature break down of callose caused male sterility in transgenic tobacco. Absent or delayed callose degradation was reported in ms2 (Graybosch, R. A., et al., Male sterility in soybean (Glycine max). I Phenotypic expression of the ms2 mutant, Am J Bot 72:1751-1764, 1985) and ms3 soybean (Buntman, D. J., et al., Microsporogenesis of normal and male sterile (ms3) mutant soybean (Glycine max), Scanning Electron Microsc 1983:913-922, 1983), in cms Capsicum (Horner, H. T. Jr., et al., A comparative light- and electron-microscopic study of microsporogenesis in amel-sterile pepper (Capsicum annuum L.), Can J Bot 52:435-441,1974), and in cms Helianthus (Horner, H. T. Jr., A comparative light-and electron-microscopic study of microsporogenesis in male-fertile and cytoplasmic male-sterile sunflower (Helianthus annuus L.), Am J Bot 64:745-759, 1977). These studies show that the timing of the callase activity is critical for normal development of microspores.
A major obstacle to F.sub.1 hybrid soybean seed production is the intensive hand-labor requirement for large numbers of pollinations.
Use of a reliable male sterility gene in soybeans, if available, could increase the seed set on female plants and would result in increased cost efficiencies and productivity of hybrid soybean seeds.